Gayness may not be in our genes, but in the molecules that regulate them. New research suggests that epigenetic factors - chemical "switches" attached to genes that turn them on or off - are a more plausible heritable mechanism behind homosexuality than DNA itself.

Non-genetic changes to gene expression are called epi-marks, for epigenetics, the field of research dealing with the molecular on/off switches. Epi-marks are normally erased between generations, but there's recent evidence that they're sometimes passed from parent to child.

Researchers at the National Institute for Mathematical and Biological Synthesis (NIMBioS) looked at how epi-marks that influence testosterone sensitivity in the womb might contribute to homosexuality. Late in pregnancy, natural variations in testosterone levels can alter a fetus' sexual development. Sex-specific epi-marks protect female fetuses from masculinisation in the presence of too much testosterone; boys are protected from feminisation if too little testosterone is present.

The search for genes that control sexual orientation is based on increasing evidence of a strong genetic component. Studies clearly show that homosexuality runs in families, with an increased rate among siblings and the maternal uncles of gay men, according to a 2011 review....